This invention relates to a hollow fiber membrane device adapted for boreside feed which possesses improved shellside countercurrent flow distribution, resulting in higher productivity while producing a high purity non-permeate product stream.
Several designs for bore-fed hollow fiber membrane devices are known. For example, see U.S. Pat. Nos. 4,871,379; 4,881,955; 4,929,259; 4,961,760; 5,013,331; and 5,013,437. In a boreside feed operation, a mixture of fluids to be separated is introduced into one end of the hollow fiber membrane device such that the fluid mixture flows down the bores (lumens) of the hollow fibers. The fluid which does not permeate through the fibers exits the bores of the fibers at the opposite ends of the fibers, while the fluid permeating through the fibers is removed from the outside of the fibers.
Conventional boreside hollow fiber membrane devices, particularly those designed for separation of gases, typically achieve substantially less than theoretical separation performance (based on membrane separation properties) due to inefficiencies in shellside flow distribution within the device. Generally, conventional bore-fed parallel-wrapped hollow fiber devices achieve only 50 to 65 percent of theoretical productivity, largely due to poor shellside flow distribution.
Conventional techniques for fabricating hollow fiber membrane devices frequently employ bias-wrapping or other methods to promote uniform packing of fibers, thus encouraging countercurrent shellside flow patterns and reducing channeling. See, for example, U.S. Pat. Nos. 3,339,341 and 3,442,002. These methods normally increase the complexity and expense of the device manufacturing process, and often result in a high degree of fiber wastage.
The performance of hollow fiber separation devices is often discussed relative to ideal performance models. Crossflow and countercurrent models are the most common. Both models assume that the fluid fed to the module flows in a direction parallel to the fibers. Because of the selective nature of the membrane, the fluid becomes progressively enriched in one or more components along the length of the fibers and progressively depleted in other components.
The ideal crossflow model assumes that the fluid permeating through the membrane at each point has a composition determined by the ratio of the fluxes through the membrane, that is, the local mole fraction of each component of the permeate is equal to the molar flux of that component through the membrane divided by the total molar flux through the membrane. The physical interpretation of this model is that all fluid permeating through the membrane is withdrawn without mixing with fluid permeating through the membrane at neighboring points.
The ideal countercurrent model assumes that the fluid permeating through the membrane collects and flows in a direction opposite to the flow of the fluid fed to the device. Because the feed fluid becomes progressively depleted in one or more components as it flows down the length of membrane device, the fluid permeating through the membrane also becomes progressively depleted in the same components. By forcing the permeating fluid to flow in a direction opposite the feed flow, the accumulated permeate dilutes the local permeate even more in the depleted components. Since the driving force for separation across the membrane is related to differences in composition, this additional dilution serves to increase the driving force and thus the separation performance of the membrane device. Consequently, ideal countercurrent performance maximizes the separating capability of a membrane device.
The equations describing ideal crossflow and countercurrent performance models are well-known to persons skilled in membrane device fabrication and mathematical methods to solve these equations are also well known.
Many factors can prevent a hollow fiber membrane device from achieving ideal countercurrent performance, including broken fibers, plugged fibers, leaks, non-uniform fiber size and/or properties, axial dispersion, and channeling. The ideal countercurrent model assumes that axial dispersion, diffusion, or mixing is negligible. The stream state (composition, pressure, temperature, and flowrate) inside each fiber is assumed to be identical at a given axial position. The flow is assumed to be perfectly countercurrent; the velocity outside the fibers depends only on the axial position and has no dependence on position perpendicular to the fibers.
Several patents describe methods for improving the performance of a device by inducing countercurrent flow patterns on the outside of the fibers to maximize the driving force for separation. Some patents discuss methods of improving the packing uniformity of the fibers to encourage even shellside flow distribution. For example, U.S. Pat. Nos. 3,339,341 and 3,442,002 describe a method for enclosing fiber bundles in porous elastic sleeves to compress the fibers, thereby increasing the fiber packing density and improving the uniformity of the spacing. This method is said to improve the flow distribution on the outside of the fibers. U.S. Pat. No. 4,308,654 describes a method of enclosing a bundle of hollow fibers in a flexible, impermeable envelope, maintaining a pressure difference across the envelope to compress the fibers together, then inserting the compressed fiber bundle into a shell. This method is also said to improve the flow distribution outside the fibers.
Several patents also discuss methods of baffling to promote countercurrent flow patterns. Outer baffles or shells with an opening at one or more ends are commonly used to force the fluid outside the fibers to flow in a generally countercurrent direction. See, for example, U.S. Pat. Nos. 4,871,379 and 4,781,834. Multiple internal baffles may also be used to promote countercurrent flow. U.S. Pat. Nos. 4,929,259, 4,961,760, and 5,013,437 describe concentric and spiral baffles in cylindrical devices with core tubes to channel flow in the countercurrent direction.
The problem with such methods of improving shellside flow distribution is that the methods of manufacturing such devices greatly increase in cost and complexity without a correspondingly significant improvement in device efficiency.
Another problem associated with conventional hollow fiber membrane devices is that such devices are typically manufactured for standardized productivities or capacities. Such standardization of device productivities frequently results in systems either over or under designed for a given customer's capacity requirements.
What is needed is a hollow fiber membrane device which is adapted for boreside feed that does not require complex manufacturing methods to obtain uniform packing of fibers, yet still achieves performance that substantially approaches theoretical countercurrent performance, including theoretical recovery. What is also needed is a hollow fiber membrane device easily manufactured for a wide range of productivities or capacities.